I did the same on my Cub Cadet RZT mower a few years back.
It has a valve on the crankcase where you connect a hose to drain the oil into a container you place underneath the mower.
I started the oil draining and figured give it a little time.
When I came back, I changed the filter, forgot to shut the valve and started adding oil.
The Kawasaki twin should have held about 2 quarts, after 3 quarts added and it still didn't measure on the stick, I figured out what I had done.
I looked, and the gallon container had overflowed and I now had 1 quart on the floor, and 4 quarts of old/new oil in the container.
So, overall a simple 2 quart change took 5 quarts and a sizable clean-up.